Shawnee Smith was pregnant during filming, but kept it a secret from everyone, including the director and producers. Her daughter gave the secret away one day during lunch, but only to the director/writer Darren Lynn Bousman.
The key hidden inside the needle pit would open the steel door in the room. Although most of the audience believe that it would lead to the exit, the director confirmed in the commentary that it would reveal two more syringes with antidote.
According to the DVD commentary, Xavier's retort to Addison was improvised each take, and got progressively more vulgar. The crew picked the line "The only door you know how to open...", thinking it was the most clever and funny.
The advertising posters for the film had to be recalled on orders from the MPAA because the two severed fingers forming the "II" in the posters did not meet guidelines for film advertising. The new poster makes it more obscure. Also, posters had listed the film as being rated R when at the time the film still hadn't gone before the MPAA for a rating.
This film originally came from a script by the director Darren Lynn Bousman which was called "The Desperate". After trying for years to get it made but being told repeatedly that it was too violent, finally a company wanted to do it because they suspected Saw (2004) which was becoming a hit at Sundance might blow out big and they wanted to capitalize on its success. Some producers even described Bousman's script as "too Saw-ish". Just before he was about to close a deal to make the movie, Saw (2004) opened huge at the box-office and the next day he received a call, and the producers asked if he could change it around to "Saw 2". Leigh Whannell (who wrote the first "Saw") was then brought on a little later to help Bousman with creating his original idea into a proper sequel to Saw (2004).
When Obi popped out of the furnace he hit Glenn Plummer's (Jonas's) eye. They had to stop filming for half an hour to make sure he was okay. He can be seen turned around in one shot, and holding his eye in another.
Jigsaw's puppet was originally controlled by fishing line (in Saw (2004)). This time around, the crew had a slightly larger budget and decided to completely redo the puppet and make it mechanically controlled.
The winner of Liongates SAW 2 Haunted House Contest went to The Mansion of Terror in Austin, TX. This haunted attraction was featured in a documentary by Jav Rivera entitled The Scare: The Mansion of Terror Documentary (2007) which went on to win several awards. Additionally, Javier was the photographer for Mansion of Terror's contest entries.
The trivia items below may give away important plot points.
The "hand trap" containing the syringe actually has a padlock with a key in it; you can see it out of focus at the top of the screen when Addison puts her hands into the trap. The filmmakers deliberately included this as a way of showing that Jigsaw knew the characters would rush in without checking the trap for a simple solution.
It can be inferred that Amanda and Daniel never needed an antidote; toward the end of the movie, while everybody else is coughing and becoming impaired, Amanda and Daniel never even breathe hard or show confusion.
Various items from the house can be seen in Jigsaw's lair when Detective Matthews is questioning him, hinting that the video feed is old. According to the director, the rope and the paddle that Amanda picked up were both in the lair, but very hard to spot. The obvious item is the picture with the "X marks the spot" on it, prominently seen behind Jigsaw in many scenes.
At the end of the film, Xavier cuts his number off the back of his neck and puts it in his pocket. On the DVD commentary, it is revealed that the script called for Xavier to merely toss the skin away, but when Franky G chose to put it in his pocket, the crew loved it so much they used it instead.
Before deciding to place Addison in the razor box trap, the creators and writers discussed (on a DVD commentary) that they had originally intended for Addison to end up in a different trap, which was similar to a waffle iron. Addison would wake up shackled at the wrists to a chair. Razors would be planted against her wrists to dig into her flesh if she tried to remove her hands, and the only way to release her hands from the razor shackles would be to burn her face against a waffle iron-like grill that was in front of her. After struggling and cutting up her wrists, Addison would then finally slam her face against the grill, releasing her cuffs and letting her go free, but with a horrendously scarred face.
Instead of cutting his eye open to get the key, Michael was originally going to be supplied with the key in one of his hands, but would be strapped to the chair at the wrists. Above the hand with the key was a razor, which would dig further into his flesh the more he reached the key up to the locks on his harness.
When Jigsaw is giving directions to the fake house, he tells Detective Matthews that it's the last house on the left. This is a reference to the movie The Last House on the Left (1972), in which the parents of a murdered teenager torture and kill the murderers.
The final used syringe found on the floor when Eric discovers the contraption door leading to the basement, was allegedly the antidote that saved Daniel's life. Amanda made sure that Daniel would not be poisoned by even the slightest dose of gas he could have inhaled inside the house.
Instead of the step shooting forward and breaking the SWAT officer's shins, a blade had originally been intended to shoot forward and slice off the first SWAT officer's legs once said rigged step had been activated.
According to the DVD commentary, the tape for Gus would have said something along the lines of him always reaching his hand into the cookie jar for money, while Addison's would talk about her using her looks to survive, stating that she would have to sacrifice and scar them if she wanted to escape. When Addison walked into the room, she found an envelope with the tape inside. Tearing it open, she found the tape for Gus, but tossed it aside, as she didn't have the tape recorder to play it.
The creators and writers had originally thought of having Amanda be forced to dig through a bathtub full of the syringes, but decided it wouldn't be enough, instead wanting it on a grander scale. After thinking of the pit in the middle of the room, they intended for Amanda to land up to her neck in the needles as if it were a pool, but realized that it would be impossible for her to sink into such a pool, along with the fact that getting enough needles would have taken too long as it took the production team a long time to get as many needles as they had.
A subplot missing from the final cut; Eric Matthews had an affair with Detective Kerry. This precipitated the end of his marriage and led to the strained relationship between him and Daniel. Moreover Eric's wife won the custody of Daniel quite easily. The only remaining hint of this back-story is when Kerry remarks that she is not going to let Eric lose his son for the second time.
There was a lot of back-story about the contestants of the game which was deleted mostly for pacing reasons. Addison was indeed a prostitute which was merely implied in the finished film. She'd had sex with Detective Matthews in the back seat of a police car but they were discovered and the scandal led to Eric's demotion. However, he made sure that Addison would be incarcerated. The first victim was called Gus and he was an embezzler of money. The needle in the Glass Box trap was intended for him with the symbolic reference of the proverb "trying to reach the cookie jar". Laura was imprisoned because she was a shoplifter. Obi was meant to reach a second portal at the back of the furnace crawling through the flames but he would be badly burned during the process accounting for the tape which stated that he would have to confront the devil.
While the creators and writers of were thinking of Gus' trap, Darren Lynn Bousman suggested a trap in which someone would have to reach into a jar. If they weren't able to retrieve the antidote, they would be forced to lose their hand. This finally developed into the razor box trap, which was then used on Addison instead.
Before settling on the final version of the furnace, the creators and writers of originally thought of having Obi's tape recorder attached to the collar of a pit bull in the basement. They decided against it, as it would lead to cruelty to animals, and instead attached it to a corpse-like mannequin. According to storyboards for this scene, Obi would have only had to reach for one antidote, while a key took the place of the second antidote.